With no other candidates remaining in the GOP race and several primaries left to go, Trump gained one vote more than the necessary 1,237 votes after several unbound delegates said they would support him at the convention, AP reported.

"I think he has touched a part of our electorate that doesn't like where our country is," Oklahoma GOP chairwoman Pam Pollard told AP of her decision to back the billionaire businessman. "I have no problem supporting Mr. Trump."

With 1,238 votes and counting, Trump now avoids a contested convention in Cleveland, even though former rivals Texas Senator Ted Cruz, Florida Senator Marco Rubio and Ohio Governor John Kasich have thus far declined to throw their support behind the presumptive nominee. All three have opted to keep their delegates as leverage for the GOP convention, Fox News reported on Tuesday.

Some unbound delegates are supporting Trump only begrudgingly. Cameron Linton, an unbound delegate from Pittsburgh, Pennsylvania, told AP he is supporting the political neophyte only because Trump won his congressional district.

"If there's a second ballot I won't vote for Donald Trump," Linton said. "He's ridiculous. There's no other way to say it."

There are still five outstanding contests on June 7, with a total of 303 delegates at stake in California, Montana, New Jersey, New Mexico and South Dakota. All but New Mexico are winner-take-all primaries.

On the Democratic side, the race is still on. Former Secretary of State Hillary Clinton has 1,768 delegates to Vermont Senator Bernie Sanders’ 1,497. However, Clinton’s lead is increased with the addition of her 537 pledged superdelegates; Sanders has only 42. The winner will need a total of 2,382 delegates and superdelegates to clinch the Democratic nomination.